First off, if anyone out there has one, I'd like to know whether or not it's worth getting a medium-format camera. You know, those 50lb "things" that give you 22MP or higher? And also, I'm looking for a really good zoom for my XTi-something like 50-450. I'm not one of the guys who knows everything about every camera and every lens *But thank god for the people who do* so I need some opinions.

If you're not sure about MF with a digital back you should probably rent one before buying. With lenses etc you're looking at $15,000+ very quickly.

Regarding zooms, that seems like a really long range... and chances are that even if such a lens was available, it wouldn't be very good. I'm also not quite the qear-head, but chances are that zoom doesn't really exist. HOWEVER - I gather there are some nice 75-300mm lenses. Check out Canon's products, but also look at Sigma and Tamron lenses.

As almost always in gear-related matters, the question is how flexible is your budget.